I had the same problem a few days ago regarding the search results by languages. I solved this cloning the Site Search Input module and adding a couple of new lines in the HTML + Hubl section and also in the JavaScript one.
For the HTML + Hubl you can add the live above underneath the values "BLOG_POST" and "LISTING_PAGE". In this way, the search will consider also the language when searching for the results.
For the JavaScript you add a new function combining it. Of course you can change the values for all the languages you have in your blog. In my case, English and German.
function languageSearch() {
if (document.documentElement.lang.includes("en")){
document.getElementById('lang-search').value = "en";
} else {
document.getElementById('lang-search').value = "de";
}
}
For the other two questions I guess the fastest solution would also be writing some similar functions in JavaScript using the same logic. Unfortunately, we don't have these options yet since it's a very recent feature from Hubspot.
The &language= parameter can be included to filter results by language. This parameter should obey the ISO639-1 format (e.g. es for Spanish), so make sure that you're including the correct language code. I've included a link to a list of ISO 639-1 codes (see below).
The &length= parameter specifies the length of the search results. Can be set to LONG or SHORT: SHORT will return a short highlight of the meta description, whereas LONG will build the snippet based on the html/content of the page.
Are you referring to the search results page? Since the search results page is a system page, customization options are limited; it's not currently possible to customize the HTML title. I'd encourage you to create an idea on the HubSpot Community's Ideas Forum for this use case.